W is for Worm
Like thing, worm is one of those little words that has accrued lots of different meanings. Obviously it can refer to a whole series of distantly-related animals. As well as true worms, for example, the word is used to refer to larvae, centipedes or even snakes.
The most familiar worms - earthworms - have been around for around 120 million years. They're an invaluable part of our ecosystem. Charles Darwin studied them at length and found that they turn over the top six inches of the soil (dirt) every twenty years. Earthworms play a vital role in maintaining soil fertility.
To a speculative fiction writer, two other senses of the word are hard to avoid. In fantasy books, "worm" is obviously another word for "dragon". Smaug is referred to as a "worm" in The Hobbit. Sometimes the word is even used to refer to the devil.
A modern meaning more likely to crop up in an SF story is worm as in a species of computer malware. In computing terms, a worm is distinct from a virus. A worm doesn't need to attach itself to some other piece of software as a virus does. It exists mainly to replicate itself across a network (such as the Internet).
The word derives from the Old English word wyrm, a word I really like.
Archaeopteryx Blunderbuss Chthonic Dreadnought Entropy Fulgurite Gargantua Humbucking Ichthyic Juggernaut Kappa Labyrinthine Megrim Numinous Ophidian Pandemonium Quark Ragnarök Shibboleth Thing Umami Verdigris Worm